Rock bolts are used for support in a variety of mining and civil engineering situations. The term ‘rock bolt’ loosely encompasses mechanical anchors, grouted anchors and friction bolts. The term ‘rock bolt’ also specifically refers to mechanical ground anchors that include threaded bolts. However, ground anchors and ground stabilizers, while often called ‘rock bolts’ may not include a bolt at all. One example is a friction stabilizer. Friction stabilizers are manufactured by a number of companies, one of which is International Rollforms, Inc. previously Ingersoll-Rand Inc., which produces a friction stabilizer under the registered trademark, “Split Set.”
A Split Set® friction stabilizer comprises a high strength, rolled steel, split tube having a tapered distal end to facilitate insertion into a drilled-out borehole and a steel ring welded adjacent to an opposite proximal end for withstanding an insertion force and for retaining a steel faceplate, see FIG. 3. A friction stabilizer is installed by pushing the split tube into a slightly undersized borehole where compression of the split tube creates a radial outward spring force onto the borehole wall, and thereby provides frictional anchorage along the length of the split tube and borehole for resisting pull out forces.
Friction stabilizers are quick and simple to install and are thus widely used by miners around the world. A rock drill typically bores a hole and impact drives the friction stabilizer into the borehole with only a quick change of work bit.
Friction stabilizers are used in a wide variety of environments, especially in tunnels and underground mines, coal to hard rock. They are often used with mesh, for example chain link fence or welded wire fabric (WWF), where a friction stabilizer faceplate holds the mesh against a rock surface, thereby catching broken rock between friction stabilizers. This is particularly useful for tunnels and mines walls and ceilings or roofs.
Corrosion can be a problem with friction stabilizers since its steel is relatively thin and in direct contact with the borehole material. Some environments are more corrosive than others, for instance coal sulfides or rock mineral deposits may contact the steel tube, or moist or alkaline salt air might enter the tube. Galvanizing friction stabilizers may help to reduce corrosion but adds to the cost and may not last for long term applications in aggressive environments.
Testing the load capacity of installed friction stabilizers is an important workplace safety measure. Earth or rock changes and corrosion can negatively affect the load carrying capacity of a friction stabilizer, often with adverse conditions hidden from sight through the depth of the borehole. Nondestructive testing methods are essential so that new and old friction stabilizers can be tested without removing or destroying the original or current load capacity.
One nondestructive method of testing is to install strain gauges on a friction stabilizer that measure movement. This type of testing involves delicate equipment in harsh mining environments, is expensive, and not practical for testing a multitude of friction stabilizer.
Another method for nondestructive testing of a friction stabilizer is to pre-install a spacer and collar on select friction stabilizers for sampling conditions. Here, the spacer and collar slide onto a split tube followed by the friction stabilizer faceplate, all of which are retained by the split tube ring flange and installed in and against the rock. For testing, a U-shaped claw is slid onto the exposed spacer and a pulling force is applied to the claw drawing down on the collar. This method requires foresight as to which friction stabilizers will be tested in the future. Moreover, mines typically use thousands of friction stabilizers, so adding a spacer and collar to each friction stabilizer is cost prohibitive, increases an inventory of parts, produces a dangerous stub protruding from a mine wall or ceiling, and shortens the insertion length of the split tube thereby decreasing load carrying capacity of the friction stabilizer. Plus, friction stabilizers have been used since about 1975, while spacers and collars were introduced long after that. Thus, older installed friction stabilizers can not be tested with this method.
Another method for nondestructive testing of an existing friction stabilizer is to attach an exterior collar and then pull on the collar. In this method, the collar has a lip which is fitted in between a friction stabilizer ring and faceplate. Unfortunately, installing the exterior collar is problematic, since rarely is there sufficient space between the friction stabilizer ring and faceplate for the collar's lip. This is particularly true when the friction stabilizer split tube is anything but ninety degrees to the faceplate, which in practice, is more often the case than not. The present FIG. 1, illustrates a ring flange 53 abutting faceplate 60 on the left side of friction stabilizer 50. When the exterior collar lip is insufficiently purchased in between a ring flange and faceplate, the collar pulls off during testing resulting in lost time, a falling equipment hazard and a failed test. Experience has shown that in use, the exterior collar testing method is slow to install, requires multiple tools, and often fails.
What is needed is a friction stabilizer pull tester apparatus capable of efficiently testing each friction stabilizer, regardless of orientation, regardless of the existence of a pre-installed collar, and regardless of age. Miners, mine operators and tunnel inspectors want more than a pull test sampling, they want to know if they are safe throughout a worksite. Furthermore, by nondestructive testing a multitude friction stabilizers, rock change patterns may be detected and thus routinely monitored for safety.